Decomposition of condensed phase energetic materials: interplay between uni- and bimolecular mechanisms

Mechanical behaviors of CL-20 under an impact loading: A molecular dynamics study

Study on the dynamic process of CL-20 crystal under impact is critical for the safe utilization of energetic materials under extreme conditions. Herein, the mechanical and structural evolution of CL-20 under the impact of a diamond ball is investigated by using molecular dynamics simulation. The considerations are given to the effect of different impact velocity, impact direction and impact angle. It is found that a high impact velocity results in a large indentation depth and force, as well as more significant energy transition and the formation of a large number of molecular fragments. Moreover, CL-20 exhibits weak anisotropy along different impact directions due to the crystalline distribution anisotropy. Furthermore, the mechanical response of CL-20 is angle-dependent, which is caused by the discrepancy in local molecular re-arrangement. These results may enhance the understanding of the mechanical behavior of CL-20 and promote its wide applications.

A Physical Organic Approach towards Statistical Modeling of Tetrazole and Azide Decomposition

Nitrogen atom-rich heterocycles and organic azides have found extensive use in many sectors of modern chemistry from drug discovery to energetic materials. The prediction and understanding of their energetic properties are thus key to the safe and effective application of these compounds. In this work, we disclose the use of multivariate linear regression modeling for the prediction of the decomposition temperature and impact sensitivity of structurally diverse tetrazoles and organic azides. We report a data-driven approach for property prediction featuring a collection of quantum mechanical parameters and computational workflows. The statistical models reported herein carry predictive accuracy as well as chemical interpretability. Model validation was successfully accomplished via tetrazole test sets with parameters generated exclusively in silico. Mechanistic analysis of the statistical models indicated distinct divergent pathways of thermal and impact-initiated decomposition.

Machine learning quantitatively characterizes the deformation and destruction of explosive molecules

Although explosives have been widely used in mines, road development, old building demolishing, and munition explosions; currently, how chemical bonds between atoms break and recombine, how the molecular structure is deformed and destroyed, how the reaction product molecules are formed, and the details for this rapid change process in explosive reactions are not yet fully understood, which limits the full use of explosive energy and safer use of explosives. This paper presents a quantitative model of molecular structure deformation using machine learning algorithms as well as a qualitative model of its relationship with molecular structure destruction, based on a molecular dynamics simulation and detailed analysis of the shock-loaded ε-CL-20, providing new perspectives for explosive community research. Specifically, the quantitative model of molecular structure deformation establishes the quantitative relationship between the molecular volume change and molecular position change, and between molecular distance change and molecular volume change using the machine learning algorithms such as Delaunay triangulation, clustering, and gradient descent. We find that the molecular spacing in explosives is strongly compressed after being shocked, and the peripheral structure can shrink inward, which is beneficial to keep the cage structure stable. When the peripheral structure is compressed to a certain extent, the cage structure volume begins to expand and is then destroyed. In addition, hydrogen atom transfer occurs within the explosive molecule. This study amplifies the structural changes and the chemical reaction process for explosive molecules after being strongly compressed by a shock wave, which can enrich the knowledge of the real detonation reaction process. The analysis method based on quantitative characterization using machine learning proposed in this study can also be used to analyze the microscopic reaction mechanism in other materials.

Theoretical Kinetic Studies on Thermal Decomposition of Glycerol Trinitrate and Trimethylolethane Trinitrate in the Gas and Liquid Phases

Glycerol trinitrate (NG) and trimethylolethane trinitrate (TMETN), as typical nitrate esters, are important energetic plasticizers in solid propellants. With the aid of high-precision quantum chemical calculations, the Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation theory and the transition state theory have been employed to investigate the decomposition kinetics of NG and TMETN in the gas phase (over the temperature range of 300-1000 K and pressure range of 0.01-100 atm) and liquid phase (using water as the solvent). The continuum solvation model based on solute electron density (SMD) was used to describe the solvent effect. The thermal decomposition mechanism is closely relevant to the combustion properties of energetic materials. The results show that the RO-NO₂ dissociation channel overwhelmingly favors other reaction pathways, including HONO elimination for the decomposition of NG and TMETN in both the gas phase and liquid phase. At 500 K and 1 atm, the rate coefficient of gas phase decomposition of TMETN is 5 times higher than that of NG. Nevertheless, the liquid phase decomposition of TMETN is a factor of 5835 slower than that of NG at 500 K. The solvation effect caused by vapor pressure and solubility can be used to justify such contradictions. Our calculations provide detailed mechanistic evidence for the initial kinetics of nitrate ester decomposition in both the gas phase and liquid phase, which is particularly valuable for understanding the multiphase decomposition behavior and building detailed kinetic models for nitrate ester.

Decomposition mechanism of 1,3,5-trinitro-2,4,6-trinitroaminobenzene under thermal and shock stimuli using ReaxFF molecular dynamics simulations

To obtain atomic-level insights into the decomposition behavior of 1,3,5-trinitro-2,4,6-trinitroaminobenzene (TNTNB) under different stimulations, this study applied reactive molecular dynamics simulations to illustrate the effects of thermal and shock stimuli on the TNTNB crystal. The results show that the initial decomposition of the TNTNB crystal under both thermal and shock stimuli starts with the breakage of the N-NO₂ bond. However, the C₆ ring in TNTNB undergoes structural rearrangement to form a C₃-C₅ bicyclic structure at a constant high temperature. Then, the C₃ and C₅ rings break in turn. The main final products of TNTNB under shock are N₂, CO₂, and H₂O, while NO,  N_2, H₂O and CO are formed instead at 1 atm under a constant high temperature. Pressure is the main reason for this difference. High pressure promotes the complete oxidation of the reactants.

Releasing chemical energy in spatiallyprogrammed ferroelectrics

Chemical energy ferroelectrics are generally solid macromolecules showing spontaneous polarization and chemical bonding energy. These materials still suffer drawbacks, including the limited control of energy release rate, and thermal decomposition energy well below total chemical energy. To overcome these drawbacks, we report the integrated molecular ferroelectric and energetic material from machine learning-directed additive manufacturing coupled with the ice-templating assembly. The resultant aligned porous architecture shows a low density of 0.35 g cm^-3, polarization-controlled energy release, and an anisotropic thermal conductivity ratio of 15. Thermal analysis suggests that the chlorine radicals react with macromolecules enabling a large exothermic enthalpy of reaction (6180 kJ kg^-1). In addition, the estimated detonation velocity of molecular ferroelectrics can be tuned from 6.69 ± 0.21 to 7.79 ± 0.25 km s^-1 by switching the polarization state. These results provide a pathway toward spatially programmed energetic ferroelectrics for controlled energy release rates.

Complete and selective nitration of tyrosine residue in peptides caused by ultraviolet matrix-assisted laser desorption/ionization

Complete and highly selective nitration of tyrosine (Tyr) as a residue-specific modification in peptides was found without side reactions, using ultraviolet matrix-assisted laser desorption/ionization (UV-MALDI) with a nitroaromatic reagent 3, 5-dinitrosalicylic acid (3,5-DNSA). The tyrosine nitration supported two propositions, namely, the UV-induced. NO₂ attack reaction mechanism by Long et al. and the C-NO₂ homolysis as a thermal process by Wiik et al. and Furman et al. With the UV-MALDI of peptides, a residue-specific reaction was observed in glycine (Gly) residue, i.e., an oxidation of the alpha-carbon of Gly due to attack of hydroxyl radical (.OH).

Coulomb Explosion Dynamics of Multiply Charged para-Nitrotoluene Cations

This work explores Coulomb explosion (CE) dissociation pathways in multiply charged cations of para-nitrotoluene (PNT), a model compound for nitroaromatic energetic molecules. Experiments using strong-field ionization and mass spectrometry indicate that metastable cations PNT²⁺ and PNT³⁺ undergo CE to produce NO₂⁺ and NO⁺. The experimentally measured kinetic energy release from CE upon formation of NO₂⁺ and NO⁺ agrees qualitatively with the kinetic energy release predicted by computations of the reaction pathways in PNT²⁺ and PNT³⁺ using density functional theory (DFT). Both DFT computations and mass spectrometry identified additional products from CE of highly charged PNT^q+ cations with q > 3. The dynamical timescales required for direct CE of PNT²⁺ and PNT³⁺ to produce NO₂⁺ were estimated to be 200 and 90 fs, respectively, using ultrafast disruptive probing measurements.

Thermo-chemo-mechanically coupled cavity dynamics and the emergence of multi-phase bursts

Understanding the dynamics induced by confined thermo-chemical processes occurring inside a solid medium is a fundamental problem of interest in several applications, such as hotspot formation and micro-explosions in energetic materials and laser-induced cavitation for energy focusing and material characterization. In recent years, advanced experimental capabilities have uncovered elusive behaviours in such systems, including temperature spikes that emerge at short timescales, and multi-phase explosions. By coupling the mechanics of the solid, the thermodynamics, and the chemical kinetics of the decomposition reactions, the theoretical model developed here explains and demonstrates these phenomena. The dimensionless response of the system is studied numerically and analytical expressions for the mechanical response limits are derived. These results should be useful in guiding future experiments and in explaining phenomena that may emerge at various length and timescales.

Unimolecular Decomposition Reactions of Picric Acid and Its Methylated Derivatives─A DFT Study

To handle energetic materials safely, it is important to have knowledge about their sensitivity. Density functional theory (DFT) has proven a valuable tool in the study of energetic materials, and in the current work, DFT is employed to study the thermal unimolecular decomposition of 2,4,6-trinitrophenol (picric acid, PA), 3-methyl-2,4,6-trinitrophenol (methyl picric acid, mPA), and 3,5-dimethyl-2,4,6-trinitrophenol (dimethyl picric acid, dmPA). These compounds have similar molecular structures, but according to the literature, mPA is far less sensitive to impact than the other two compounds. Three pathways believed important for the initiation reactions are investigated at 0 and 298.15 K. We compare the computed energetics of the reaction pathways with the objective of rationalizing the unexpected sensitivity behavior. Our results reveal a few if any significant differences in the energetics of the three molecules, and thus do not reflect the sensitivity deviations observed in experiments. These findings point toward the potential importance of crystal structure, crystal morphology, bimolecular reactions, or combinations thereof on the impact sensitivity of nitroaromatics.

Fluorescein based fluorescent and colorimetric sensors for sensitive detection of TNP explosive in aqueous medium: Application of logic gate

Rapid detection of 2,4,6-trinitrophenol (TNP) in real samples has recently attained considerable attention from the perspective of national security, human health, and environmental safety. In this context, cost-effective and convenient detection of TNP explosive was accomplished through two new fluorescein based sensors F2 and F3. Sensors displayed effective fluorescence quenching response towards TNP in the aqueous medium. Highly sensitive fluorescence detection of TNP explosive (detection limit, 0.73 (F2) and 1.7 nM (F3)) was governed by ground-state charge transfer complex formation, facilitated by favorable H-bonding between sensor and TNP explosive. Fluorescence quenching mechanism for the detection of TNP explosive was investigated through UV-Visible absorption, dynamic light scattering (DLS), density functional theory (DFT) calculations, the Benesi-Hildebrand, and Job's plots. Advantageously, sensors displayed selective and immediate colorimetric recognition of TNP explosive. Importantly, sensors exhibited quick response time towards TNP even in the presence of potential interferences that make them highly suitable for practical applications. Sensors were successfully applied for fluorescent and colorimetric detection of TNP explosive in industrial water samples and fabrication of logic gates. Further, convenient contact mode and instant surface sensing of TNP explosive were achieved through the fabrication of fluorescent strips and explosive responsive test kits.

Reactive Molecular Dynamics Calculation and Ignition Delay Test of the Mixture of an Additive and 2-Azido-N,N-dimethylethanamine with Dinitrogen Tetroxide

In order to shorten the ignition delay of 2-azido-N,N-dimethylethanamine (DMAZ) and dinitrogen tetroxide (NTO), four amines [tert-butylamine, pyrrole, N,N,N',N'-tetramethyl ethylenediamine (TMEDA), and diethylenetriamine (DABH)] with a mass fraction of 5% were added to DMAZ, and the potential energy change and the product change during the reaction of the mixture of an additive and DMAZ with NTO were analyzed by Reactive molecular dynamics (ReaxFF MD) calculation. Then, the ignition delay of the mixture of the additive and DMAZ as well as pure DMAZ with NTO was measured by a drop experiment with a photoelectric sensor and high-speed camera. The results show that the addition of pyrrole greatly reduced the time to reach the maximum system energy and greatly increased the rate of HNO₂ formation. The dripping of the fuel was approximately a uniform linear motion, and the expression was y = 43.13 + 7.16x. The ignition delay time recorded by the camera was in good agreement with that of the optical signal. The measured ignition delay time for DMAZ with NTO was 261.5 ms. The mixture of pyrrole and DMAZ with NTO had the shortest ignition delay time of 100 ms, and the proportion of shortening the ignition delay time was the largest. The results of the droplet experiment were consistent with those of ReaxFF MD simulation, indicating that HNO₂ plays an important role in the ignition delay, that is, the formation rate of HNO₂ is positively correlated with the ignition delay.

Pressure and Polymorph Dependent Thermal Decomposition Mechanism of Molecular Materials: A Case of 1,3,5,-Trinitro-1,3,5,-triazine

1,3,5,-Trinitro-1,3,5,-triazine (RDX) serves as an important energetic material and is widely used as various solid propellants and explosives. Understanding the thermal decomposition behaviors of various polymorphs of RDX at high pressure and high temperature is significantly important for safe storage and handling. The present work reveals the early thermal decay mechanisms of two polymorphs (α- and ε-forms) of RDX at high pressure and high temperature by ReaxFF reactive molecular dynamic simulations and climbing image nudged elastic band (CI-NEB) static calculations. It is found that the thermal decomposition rate has positive and negative effects on the pressure for α- and ε-RDX, respectively. This difference originates from the difference of pressure effect on the intermolecular H transfer of the two polymorphs, as we confirm that the bimolecular H transfer rather than the NO₂ partition initiates the decay with a significantly lower energy barrier therein. This finding may be beneficial to understand the pressure and polymorph dependent effect on the decay of RDX and to develop a kinetic model for the combustion of solid RDX.

Pressure effects on the thermal decomposition of the LLM-105 crystal

Thermal mechanical responses under high temperature and high pressure are basic information to understand the performance of energetic materials. In this work, the pressure effects on the thermal decay of 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105) are explored. Up to the initial pressure of 4.6 GPa, the pressure dependent decomposition boundary is built and no phase transition occurs until the decomposition of the LLM-105 crystal. The decomposition temperature is significantly lifted via a weak loading pressure. The experimental measurement confirms the decomposition products, including NO₂, CO₂ and NH₃, which are predicted by the density functional tight-binding molecular dynamics method. The calculation described the details of thermal decay in the initial stages under high pressure. The sudden drop in the shifts of the Raman modes associated with hydrogen bonds under high pressure indicates the strengthening of the intermolecular hydrogen bonds and the occurrence of intermolecular hydrogen transfer prior to crystal decomposition. The simulation supported the existence of intermolecular hydrogen transfer and provided the transfer path and decomposition mechanism. All of these jobs not only contribute significantly to the understanding of thermal decomposition of energetic materials as a function of pressure, but also contribute to the understanding of sensitivity mechanisms and safety issues.

Fluorene-Based Fluorometric and Colorimetric Conjugated Polymers for Sensitive Detection of 2,4,6-Trinitrophenol Explosive in Aqueous Medium

Nitroaromatic explosives are a class of compounds that are responsible for various health hazards and terrorist outrages. Among these, sensitive detection of 2,4,6-trinitrophenol (TNP) explosive has always been highly desirable considering public health and national security. In this regard, three fluorene-based conjugated polymers (CP 1, CP 2, and CP 3) were synthesized through the Suzuki-Miyaura coupling reaction and were found to be highly sensitive for fluorescence detection of TNP with detection limits of 3.2, 5.7, and 6.1 pM, respectively. Excellent selectivity of CPs toward TNP was attributed to their unique π-π interactions based on fluorescence studies and density functional theory (DFT) calculations. The high sensitivity of CPs to TNP was attributed to the static quenching mechanism based on the photoinduced electron transfer process and was evaluated by fluorescence, UV-visible absorption, dynamic light scattering, Job's plots, the Benesi-Hildebrand plots, and DFT calculations. CPs were also used for colorimetric and real-water sample analysis for the detection of TNP explosive. Meanwhile, sensor-coated test strips were fabricated for on-site detection of TNP, which makes them convenient solid-supported sensors.

Molecular dynamic insight into octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and the nano-HMX decomposition mechanism

The entire decomposition reaction process of a 30 Å HMX nanoparticle at 2000 K by ReaxFF molecular dynamics.

Early thermal decay of energetic hydrogen- and nitro-free furoxan compounds: the case of DNTF and BTF

Exploration of the initial reactions of H-free and nitro-free energetic materials could enrich our understanding of the thermal decomposition mechanism of various energetic materials (EMs). In this work, two furoxan compounds, 3,4-dinitrofurazanfuroxan (DNTF) and benzotrifuroxan (BTF), were investigated to shed light on the decay mechanism of furoxan compounds based on the combination of self-consistent charge density functional tight binding and molecular dynamics simulations. The results show that DNTF and BTF decay via a unimolecular mechanism, and the transformation of the furoxan ring into a nitro group is suggested as a novel initial channel. Five initial steps of DNTF thermal decomposition are observed, including NO₂ loss and the N(O)-O bond cleavage of the central and peripheral rings. The bond cleavage of peripheral rings dominates the decay at low temperatures, while the central ring opening and C-NO₂ dissociation govern the high temperature decay. Besides, NO₂, CO and NO fragments are mainly yielded at high temperatures, while CO₃N₂ is dominant at low temperatures. The three-stage characteristic of the exothermic BTF decay is described under programmed heating conditions for the first time. Four initial steps of BTF thermal decomposition were identified, including furoxan ring opening reactions and the breakage of the 6-membered ring C-C bond. The cleavage of the N(O)-O bond is dominant in the initial step of BTF decomposition under different heating conditions, and the frequency increases with increasing temperature. In addition, the amounts of CON, ON and CO are higher at high temperatures, while C₂O₂N₂ shows an opposite trend. The findings of this work provide deep insights into the complicated sensitivity mechanism of EMs.

Nature of the Trigger Linkage in Explosive Materials Is a Charge-Shift Bond

Explosion begins by rupture of a specific bond, in the explosive, called a trigger linkage. We characterize this bond in nitro-containing explosives. Valence-bond (VB) investigations of X-NO₂ linkages in alkyl nitrates, nitramines, and nitro esters establish the existence of Pauli repulsion that destabilizes the covalent structure of these bonds. The trigger linkages are mainly stabilized by covalent-ionic resonance and are therefore charge-shift bonds (CSBs). The source of Pauli repulsion in nitro explosives is unique. It is traced to the hyperconjugative interaction from the oxygen lone pairs of NO₂ into the σ_(X-N)^* orbital, which thereby weakens the X-NO₂ bond, and depletes its electron density as X becomes more electronegative. Weaker trigger bonds have higher CSB characters. In turn, weaker bonds increase the sensitivity of the explosive to impacts/shocks which lead to detonation. Application of the analysis to realistic explosives supports the CSB character of their X-NO₂ bonds by independent criteria. Furthermore, other families of explosives also involve CSBs as trigger linkages (O-O, N-O, Cl-O, N-I, etc. bonds). In all of these, detonation is initiated selectively at the CSB of the molecule. A connection is made between the CSB bond-weakening and the surface-electrostatic potential diagnosis in the trigger bonds.

Thermal Decomposition Mechanism and Energy Release Law of Novel Cyclo-N5--Based Nitrogen-Rich Energetic Salt

Detonation energy of novel cyclo-N₅^--based nitrogen-rich energetic salts is expected to exceed 3 times the equivalent of TNT. PHAC([(N₅)₆(H₃O)₃(NH₄)₄Cl]) was selected as the prototype to investigate the thermal decomposition reaction of PHAC in the solid phase for the first time by the first-principles molecular dynamics method. At about 38 ps, the final state of the reaction was reached. It was found that there were mainly five final products, among which the proportion of N₂ molecules was the maximum and accounted for 60% (mole fraction) of all final products. The reaction pathways of PHAC were analyzed, and more than 30 elementary reactions were found. The initial reaction of the PHAC thermal decomposition was the ring-opening of cyclo-N₅^- ion and proton transfer. The energy release of PHAC thermal decomposition is divided into two stages. The first stage is a slow release of energy before the formation of the HN₃ molecule. The second stage is the rapid release of energy after the formation of HN₃ molecules. The HN₃ molecule is an essential junction, and the unimolecular dissociation of HN₃ is the rate-determining step. Such an understanding of reaction mechanism and energy release law greatly promotes the application and synthesis of novel cyclo-N₅^--based nitrogen-rich energetic salts.

Mechanism of the improvement of the energy of host-guest explosives by incorporation of small guest molecules: HNO3 and H2O2 promoted C-N bond cleavage of the ring of ICM-102

Host–guest materials exhibit great potential applications as an insensitive high-energy–density explosive and low characteristic signal solid propellant. To investigate the mechanism of the improvement of the energy of host–guest explosives by guest molecules, ReaxFF-lg reactive molecular dynamics simulations were performed to calculate the thermal decomposition reactions of the host–guest explosives systems ICM-102/HNO₃, ICM-102/H₂O₂, and pure ICM-102 under different constant high temperatures and different heating rates. Incorporation of guest molecules significantly increased the energy level of the host–guest system. However, the initial reaction path of the ICM-102 molecule was not changed by the guest molecules. The guest molecules did not initially participate in the host molecule reaction. After a period of time, the H₂O₂ and HNO₃ guest molecules promoted cleavage of the C–N bond of the ICM-102 ring. Stronger oxidation and higher oxygen content resulted in the guest molecules more obviously accelerating destruction of the ICM-102 ring structure. The guest molecules accelerated the initial endothermic reaction of ICM-102, but they played a more important role in the intermediate exothermic reaction stage: incorporation of guest molecules (HNO₃ and H₂O₂) greatly improved the heat release and exothermic reaction rate. Although the energies of the host–guest systems were clearly improved by incorporation of guest molecules, the guest molecules had little effect on the thermal stabilities of the systems.

Predicted Reaction Mechanisms, Product Speciation, Kinetics, and Detonation Properties of the Insensitive Explosive 2,6-Diamino-3,5-dinitropyrazine-1-oxide (LLM-105)

2,6-Diamino-3,5-dinitropyrazine-1-oxide (LLM-105) is a relatively new and promising insensitive high-explosive (IHE) material that remains only partially characterized. IHEs are of interest for a range of applications and from a fundamental science standpoint, as the root causes behind insensitivity are poorly understood. We adopt a multitheory approach based on reactive molecular dynamic simulations performed with density functional theory, density functional tight-binding, and reactive force fields to characterize the reaction pathways, product speciation, reaction kinetics, and detonation performance of LLM-105. We compare and contrast these predictions to 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), a prototypical IHE, and 1,3,5,7-tetranitro-1,3,5,7-tetrazoctane (HMX), a more sensitive and higher performance material. The combination of different predictive models allows access to processes operative on progressively longer timescales while providing benchmarks for assessing uncertainties in the predictions. We find that the early reaction pathways of LLM-105 decomposition are extremely similar to TATB; they involve intra- and intermolecular hydrogen transfer. Additionally, the detonation performance of LLM-105 falls between that of TATB and HMX. We find agreement between predictive models for first-step reaction pathways but significant differences in final product formations. Predictions of detonation performance result in a wide range of values, and one-step kinetic parameters show the similar reaction rates at high temperatures for three out of four models considered.

Chemical reactions of a CL-20 crystal under heat and shock determined by ReaxFF reactive molecular dynamics simulations

Studying the chemical reactions of hexanitrohexaazaisowurtzitane (CL-20) under heat and shock is helpful to understand its sensitivity and shock initiation mechanism. In this work, several molecular dynamics simulations were performed under three different conditions: high temperature, high temperature and pressure, and shock. The formation and breakage of chemical bonds, changes of bond lengths, and initial reactions were analysed. It was found that the main small-molecule product of CL-20 during initial decomposition under the three different conditions was always NO2, but the generation pathways were different. At high temperatures, NO2 was generated by the direct cleavage of N-NO2 bonds. In contrast, high pressure and shock promoted the transfer of O atoms to N atoms connected to NO2, leading to the breakage of N-NO2 bonds. Almost all NO2 originated from the transfer of O atoms under the shock conditions.

Application of a multi-channel in-situ infrared spectroscopy: The case of LLM-105

The thermal decomposition process of 2,6-diamino-3,5-dinitropyrazine-1-oxide(LLM-105)under several constant temperatures (100 °C, 115 °C, 130 °C, and 145 °C) have been studied by a multi-channel in-situ reaction system. Almost 1000 spectra were obtained within 24 days by Fourier-transform infrared spectroscopy (FT-IR). The thermal decomposition activation energies (E_α) of C-NH₂ and C-NO₂ in LLM-105 were calculated by the Arrhenius equation to be 89.65 and 145.09 kJ mol^-1, respectively. The thermal decomposition process of LLM-105 under long-term constant temperature is divided into two paths: intramolecular H-transfer and C-NO₂ partition. It is feasible to study the aging process of materials using a combination of a multi-channel in-situ reaction system and FT-IR, which can effectively monitor the evolution of structure.

The thermal decomposition process of Composition B by ReaxFF/lg force field

Composition B is a melt-cast explosive consisting of mixtures of TNT and RDX. It has many excellent properties, but there are still multiple safety problems when it is used. Therefore, it is of importance to understand the thermal decomposition mechanism of Composition B. In this paper, during the establishment of the supercell model, the mass ratio of TNT to RDX is about 2:3, which accords with the actual proportion of formula of Composition B. Afterward, the thermal decomposition reaction of Composition B is conducted at various temperatures (2000 K, 2500 K, 3000 K, 3500 K, and 4000 K) by using molecular dynamics simulation of ReaxFF/lg. In terms of potential energy (PE) evolution, primary reaction, intermediate product, final product, and clusters, the thermal decomposition mechanism of Composition B is made an analysis. The activation energy of Composition B is 141.8 kJ/mol by fitting the kinetic parameters of the reaction. During the decomposition process of Composition B, the decay rate of RDX is faster than that of TNT, and the decay rates of TNT and RDX is accelerated significantly with the increasing temperature. The higher the temperature, the shorter the time difference between the two to fully decompose. It can be revealed from the result that the initial reaction path of Composition B decomposition is N-NO₂ of RDX cleavage to form NO₂, followed by the reaction of TNT with NO₂ and other molecules. The initial decomposition reaction path of Composition B is the similar at different temperatures. The main products are small molecules (NO₂, NO, N₂O, H₂O, CO₂, N₂, H₂, HNO₂, and HNO). Temperature can make a great difference for the structure of clusters. Large clusters in the system will break down into smaller molecules at high temperature.

A mechanism for two-step thermal decomposition of 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105)

2,6-Diamino-3,5-dinitropyrazine-1-oxide (LLM-105) is a representative of the new generation of low-sensitivity energetic materials and has been applied extensively in formulations as an insensitive high-energetic ingredient. Although the initial thermal decomposition mechanism of LLM-105 has been studied based on quantum chemical calculations, the internal mechanism of the two-step thermal decomposition still lacks experimental research. Thus, this study involves a detailed experimental study to reveal the mechanism of the two-step thermal decomposition of LLM-105. The results showed that LLM-105 decay was a consecutive reaction. The first-step reaction dominated the early stage of the LLM-105 decomposition, and its products participated in the reaction of the second step. The cleavage of NO2 and NH2 groups of LLM-105 mainly occurred in the first step, while gaseous products NO and C2N2 were released during the second reaction step. The first-step reaction had a higher oxygen consumption rate and a lower carbon consumption rate, producing more heat due to more extensive oxidation of the carbon backbone. The difference in the oxidative ability and reaction rate between the two steps resulted in a two-step exothermic and mass loss behavior. This study provides further insights into the entire reaction process of LLM-105 and would be helpful for its better application and for the design of new explosives.

Study of the thermal decomposition mechanism of FOX-7 by molecular dynamics simulation and online photoionization mass spectrometry

The thermal decomposition mechanism of energetic materials is important for analyzing the combustion mechanisms of propellants and evaluating the safety of propellants during transport and storage. 1,1-Diamino-2,2-dinitroethylene (FOX-7) is an important insensitive energetic material that can be used as an oxidizer in propellants. However, the initial decomposition mechanism of FOX-7 is not clear to date. The ReaxFF molecular dynamics method is widely used in the investigation of the thermal decomposition mechanisms of energetic materials. Meanwhile, the combination of thermogravimetry with online photoionization time-of-flight mass spectrometry (TG-PI-TOF-MS) and online single-photon ionization time-of-flight mass spectrometry (SPI-TOF-MS) can reveal the decomposition products, which may be integrated with the results of the simulation. In this study, the primary thermal decomposition mechanism of 1,1-diamino-2,2-dinitroethylene (FOX-7) was studied by the ReaxFF molecular dynamics simulations and online photoionization mass spectrometry. The results of the molecular dynamics simulations showed that the primary decomposition step of FOX-7 is C–NO₂ cleavage; after this, C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O formation occurs via a three-membered ring transition state, followed by NO elimination. The remaining structure loses NH₂ and H, resulting in the formation of the NHCCO structure, which finally breaks down into HNC and CO. NH₂ reacts with an H atom to produce NH₃. A reversible intramolecular hydrogen transfer was also observed at 2500 K; however, it failed to dominate the decomposition reaction. During the decomposition of FOX-7, the major products are N₂, NH₃, CO₂, and H₂N₂ and the minor products are H₂O, HN₂, and H₂. The TG-PI-TOF-MS spectrum shows three signals, i.e., m/z = 18, 28, and 30, which can be assigned to H₂O, CO, and NO, respectively. Moreover, four signals at m/z = 72.72, 55.81, 45.79, and 29.88 corresponding to the products (NH₂)₂CCO, (NH₂)CCO, NO₂, and NO have been obtained in the SPI-TOF-MS spectrum. The experimental data obtained via online photoionization mass spectrometry further validated the results of the molecular dynamics simulations.

In this work, the primary thermal decomposition mechanism of 1,1-diamino-2,2-dinitroethylene (FOX-7) was studied by ReaxFF molecular dynamics simulations and online photoionization mass spectrometry.

Reactive molecular dynamics simulation of the high-temperature pyrolysis of 2,2',2'',4,4',4'',6,6',6''-nonanitro-1,1':3',1''-terphenyl (NONA)

2,2′,2′′,4,4′,4′′,6,6′,6′′-Nonanitro-1,1′:3′,1′′-terphenyl (NONA) is currently recognized as an excellent heat-resistant explosive. To improve the atomistic understanding of the thermal decomposition paths of NONA, we performed a series of reactive force field (ReaxFF) molecular dynamics simulations under extreme conditions of temperature and pressure. The results show that two distinct initial decomposition mechanisms are the homolytic cleavage of the C–NO₂ bond and nitro–nitrite (NO₂ → ONO) isomerization followed by NO fission. Bimolecular and fused ring compounds are found in the subsequent decomposition of NONA. The product identification analysis under finite time steps showed that the gaseous products are CO₂, N₂, and H₂O. The amount of CO₂ is energetically more favorable for the system at high temperature or low density. The carbon-containing clusters are a favorable growth pathway at low temperatures, and this process was further demonstrated by the analysis of diffusion coefficients. The increase of the crystal density accelerates the decomposition of NONA judged by the analysis of reaction kinetic parameters and activation barriers. In the endothermic and exothermic stages, a 20% increase in NONA density increases the activation energies by 3.24 and 0.48 kcal mol⁻¹, respectively. The values of activation energies (49.34–49.82 kcal mol⁻¹) agree with the experimental data in the initial decomposition stage.

The bimolecular and fused ring compounds are found in the high-temperature pyrolysis of NONA using ReaxFF molecular dynamics simulations.

Decomposition mechanism scenarios of CL-20 co-crystals revealed by ReaxFF molecular dynamics: similarities and differences

Understanding the similarities and differences of decomposition mechanisms of CL-20 and its cocrystals is of great interest for practical applications of CL-20 cocrystals. The responses of CL-20 cocrystals to thermal stimulus were investigated using ReaxFF molecular dynamics simulations of two representative cocrystals, CL-20/HMX and CL-20/TNT, under adiabatic conditions and comparing to the baseline system of pure CL-20. The comprehensive chemical details were revealed with the aid of the unique code of VARxMD. The three CL-20-involved reactive systems all exhibit a distinct three-stage character during adiabatic decomposition when using the double peaks of the major intermediate NO₂ amount as the boundary. By taking advantage of the three-stage classification, a clear scenario for the similar stimulus-response of the CL-20 cocrystals can be elucidated for the dominant primary decomposition of CL-20 in stage I and the transition of favored chemical mechanisms from the generation of intermediates/radicals in stage II into their consumption to form stable products in stage III. The similar chemical behaviors are rooted in the dominance of CL-20 chemistry in the initial response of its cocrystals to thermal stimulus. The prolonged reaction zone uncovers the slowed decomposition kinetics of CL-20/HMX and CL-20/TNT, which is associated with the altered kinetics of CL-20 decomposition specifically by N-NO₂ bond scission and CL-20 skeleton decay. The retarded CL-20 decomposition in its cocrystals consequently results in more moderate self-heating and less violent exothermic reactions that agrees with the experimental observations of improved stability and damaged detonation performance of CL-20 cocrystals, particularly for CL-20/TNT. The results obtained in this work suggest that ReaxFF MD simulations can provide useful insight for the modulated chemical properties of varied CL-20 cocrystals.

Thermal stability mechanism via energy absorption by chemical bonds bending and stretching in free space and the interlayer reaction of layered molecular structure explosives

Layered molecular structure explosives have the characteristic of great thermal stability.

Transforming the Accuracy and Numerical Stability of ReaxFF Reactive Force Fields

Molecular dynamics (MD) simulations provide an important link between theories and experiments. While ab initio methods can be prohibitively costly, the ReaxFF force field has facilitated in silico studies of chemical reactivity in complex, condensed-phase systems. However, the relatively poor energy conservation in ReaxFF MD has either limited the applicability to short time scales, in cases where energy propagation is important, or has required a continuous coupling of the system to a heat bath. In this study, we reveal the root cause of the unsatisfactory energy conservation, and offer a straightforward solution. The new scheme results in orders of magnitude improvement in energy conservation, numerical stability, and accuracy of ReaxFF force fields, compared to the previous state-of-the-art, at no additional cost. We anticipate that these improvements will open new avenues of research for more accurate reactive simulations in complex systems on long time scales.

Mechanisms and kinetics of initial pyrolysis and combustion reactions of 1,1-diamino-2,2-dinitroethylene from density functional tight-binding molecular dynamics simulations

The density functional tight-binding molecular dynamics approach was used to study the mechanisms and kinetics of initial pyrolysis and combustion reactions of isolated and multi-molecular FOX-7. Based on the thermal cleavage of bridge bonds, the pyrolysis process of FOX-7 can be divided into three stages. However, the combustion process can be divided into five decomposition stages, which is much more complex than the pyrolysis reactions. The vibrations in the mean temperature contain nodes signifying the formation of new products and thereby the transitions between the various stages in the pyrolysis and combustion processes. Activation energy and pre-exponential factor for the pyrolysis and combustion reactions of FOX-7 were obtained from the kinetic analysis. It is found that the activation energy of its pyrolysis and combustion reactions are very low, making both take place fast. Our simulations provide the first atomic-level look at the full dynamics of the complicated pyrolysis and combustion process of FOX-7.

Simplifying the Electrolyte Systems with the Functional Cosolvent

The state-of-the-art electrolytes utilized in lithium-ion batteries are based on liquid carbonates combining a number of additives to fulfill the practical requirements including safety and low temperature. The plenty of components result in the quadruple times of probable radical groups involved into the interfacial reactions, rendering it too difficult to control the surface layer. This work tends to simplify the system with the fluorine-substituted ether as the functional cosolvent to expand the functions of basic electrolytes. The incorporation of this solvent enables the electrolyte to self-extinguish, reduces its freezing point to ∼75 °C lower, and assists in the formation of LiF-rich protective interlayers, resulting in the improvement of the rate capability, cryogenic performance, and cyclic stability for the LiNi_1/3Co_1/3Mn_1/3O₂ cathode. This novel design could significantly diminish the amount of necessary additives and possess the acceptable cost, which provides a probability to revitalize the development of liquid electrolytes.

Decomposition Mechanism of Hexanitrohexaazaisowurtzitane (CL-20) by Coupled Computational and Experimental Study

A novel degradation pathway of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) was identified using computational and experimental methods. Density functional theory (DFT) calculations were employed to obtain its unimolecular degradation pathway, and ultrahigh-performance liquid chromatography-high-resolution mass spectrometry, thermogravimetry-Fourier transform infrared spectrometry, thermogravimetry, and differential scanning calorimetric experimental data were used to validate the computationally deduced degradation pathways. Based on the indications from computational and experimental results, the cleavage of the strained fragment from CL-20 was identified instead of NO₂ or HONO elimination as in conventional high energy materials. This fragmentation results in the formation of two energetic species, dinitrodihydropyrazine and dinitroformimidamide, which makes CL-20 one of the most powerful energetic materials. This novel degradation pathway of CL-20 will be useful in understanding the decomposition of cage molecules, design of new practical energetic molecules, and development/improvement of thermokinetic codes used for energetic property calculations.

Optical limiting properties of surface functionalized nanodiamonds probed by the Z-scan method

This work focuses on the optical limiting behavior of surface modified nanodiamonds (DNDs) namely, amino-terminated DNDs (DND-NH2) and hydrogen-terminated DNDs (DND-H). Their relevant nonlinear optical properties for optical limiting are compared to those of unfunctionalized DNDs. The optical limitation is characterized by means of nonlinear transmittance, Z-scan, and scattered intensity assessments when submitted to a nanosecond pulsed Nd:YAG laser operating at a wavelength of 532 nm. It is stated that the largest nonlinear attenuation is attributed to the DND-H system, whereas the exceedingly low threshold values for optical limiting for the DND-H and the DND-NH2 systems is attributed to their negative electron affinity character (NEA). Using Z-scan experiments, it is shown that nonlinear refraction combined with a significant nonlinear absorption predominates in the DND-H suspension, while the pure thermal origin of the nonlinear refractive index change is conjectured in the case of the DNDs. Besides, an amazing valley to peak profile was measured on DND - NH2indicating an unexpected positive sign of the nonlinear refraction coefficient. In addition, a stronger backscattered intensity signal is highlighted for the unfunctionalized DNDs through nonlinear scattering measurements.

Thermal Stability and Detonation Properties of Potassium 4,4'-Bis(dinitromethyl)-3,3'-azofurazanate, an Environmentally Friendly Energetic Three-Dimensional Metal-Organic Framework

Environmentally acceptable alternatives to toxic lead-based primary explosives have become increasingly demanding for energetic materials (EMs) because of environmental concerns. Recent experiments suggested that energetic three-dimensional (3D) metal-organic frameworks (MOFs) are promising candidates for the next generation of environmentally friendly primary explosives. A new energetic 3D MOF, denoted as potassium 4,4'-bis(dinitromethyl)-3,3'-azofurazanate, was synthesized and suggested as an excellent candidate for green primary explosives. To achieve an atomistic-level understanding of the thermal stability and detonation properties of this material, we carried out quantum mechanics simulations to examine its initial decomposition mechanism and the Chapman-Jouguet state for sustainable detonation. We find that the initial decomposition reaction of potassium 4,4'-bis(dinitromethyl)-3,3'-azofurazanate is to break the C₂N₂O five-member ring in which K⁺ ions play a significant role in stabilizing the molecule structure, leading to an excellent thermal stability. Furthermore, this MOF system has a higher detonation velocity than that of lead azide, a comparable detonation pressure and temperature, and no toxic gases are produced at detonation. The combination of these detonation properties makes it a promising candidate for green EMs. Our results suggest that synthesizing 3D MOFs is an effective approach to develop environmentally acceptable alternatives to toxic EMs with enhanced thermal stability.

Reversible intramolecular hydrogen transfer: a completely new mechanism for low impact sensitivity of energetic materials

The intramolecular H transfer of energetic NO₂-compounds has been recognized as a possible primary step in triggering molecular decomposition for a long time.

Adiabatic and constant volume decomposition process of condensed phase δ-1,3,5,7-tetranitro-1,3,5,7-tetrazocane at high temperatures: Quantum molecular dynamics simulations

We performed quantum molecular dynamics simulations to investigate the initiation chemistry of condensed phase δ-HMX at high temperatures by maintaining constant energy and volume to model adiabatic initiation process. The decomposition of HMX began by the C-N bond breaking in one molecule and by the C-H bond cleavage in other HMX molecule at 2400 K. At 2700 K, HMX is triggered by only one path that the C-N bond broke and the ring opened. At 3000 K, the decomposition of HMX is triggered by the C-H bond and N-O bond fission in the branch chains. There are seven decomposition channels observed during the whole decomposition stage. The N-O bond cleavage is a dominant reaction pathway. The boat configuration of the HMX molecule caused a new reaction channel to be happened by forming a new N-N bond. Another new reaction channel took place to form a new N-C bond due to intermolecular effects.

Automated Chemical Kinetic Modeling via Hybrid Reactive Molecular Dynamics and Quantum Chemistry Simulations

An automated scheme for obtaining chemical kinetic models from scratch using reactive molecular dynamics and quantum chemistry simulations is presented. This methodology combines the phase space sampling of reactive molecular dynamics with the thermochemistry and kinetics prediction capabilities of quantum mechanics. This scheme provides the NASA polynomial and modified Arrhenius equation parameters for all species and reactions that are observed during the simulation and supplies them in the ChemKin format. The ab initio level of theory for predictions is easily exchangeable, and the presently used G3MP2 level of theory is found to reliably reproduce hydrogen and methane oxidation thermochemistry and kinetics data. Chemical kinetic models obtained with this approach are ready to use for, e.g., ignition delay time simulations, as shown for hydrogen combustion. The presented extension of the ChemTraYzer approach can be used as a basis for methodological advancement of chemical kinetic modeling schemes and as a black-box approach to generate chemical kinetic models.

Simulating the unimolecular decomposition pathways of cyclotrimethylnitramine (RDX) : Decomposition pathways of RDX

Based on the three known proposed pathways for the uni-molecular decomposition of RDX, we have formulated the rate equations. A kinetic Monte Carlo code has been developed and used to simulate the uni-molecular decomposition of RDX based on these equations. The KMC simulations allow one to explore each of the decomposition pathways individually and also the three competing pathways at a specified temperature and pressure. The pressure dependence is incorporated using Lindemann's formalism. The code is validated by reproducing the species evolution along each pathway. Amongst the three proposed pathways, the most likely path of RDX decomposition and the time evolution of various molecular species at different ambient temperatures and pressures are obtained. An analytical model has been developed to reproduce the decomposition pathways, which matches the simulation results.